Drilling systems are frequently used to provide cylindrical holes in metallic workpieces. The cutting or boring action of the drill system may be carried out by an elongated, substantially cylindrical drilling tool, such as a combination of a tool holder and a spade drill insert, which is selectively attached thereto. Such an arrangement may then be used in an application wherein one end of the tool holder is securely mounted in a driving apparatus, which rotates the holder about its longitudinal axis. At the opposite end of the elongated tool holder, the cutting insert engages the material to be cut. Alternatively, the workpiece may be made to rotate relative to the holder and cutting insert, such as in positioning the holder in the tail stock of a lathe or the like. Further, the tool and workpiece may be made to rotate relative to one another. The use of spade drill cutting inserts allows for quick changing of the insert upon wear of the cutting surfaces instead of the entire tool, and allows for one tool to be used for a variety of different boring applications by simply changing the insert and not the entire drill assembly.
Spade drill inserts are characterized in that they are generally flat having a pair of connection surfaces or faces on opposite sides of the insert that are parallel to each other and provide a register surface for the clamping arms of the holder. The leading end of the spade drill typically has a pair of transverse cutting edges in the form of an obtuse V. The trailing end or base of the spade drill insert is typically planar and perpendicular to the pair of connection surfaces and engages the bottom of the holder slot. The sides across the width of the insert, typically referred to as the lands, comprise the margin adjacent the leading connection surface and a clearance adjacent the trailing connection surface of the insert. The margin is cylindrically formed about the rotational axis of the spade drill and engages the sides of a hole that is being cut. The width of the margin is typically about one quarter to one third of the width of the insert and is formed parallel to the rotational axis of the spade drill on the cutting edge side of the width. The remainder of each side is the clearance surface which is radially inward of the margin such that it does not contact the hole being machined.
One problem with prior art spade drill inserts is that only a small radial portion of the width side is in contact with the hole which reduces the stability of the spade drill and may cause excessive exit chatter when cutting select materials. Another problem with some prior art spade drill inserts having large margins is that the margin surface area rubs against the side of the hole as the spade drill rotates which causes additional heat build up on the insert and may require reduced cutting speeds and may also shorten the life of the insert.